In many technical systems controllers are used to bring the process metrics of all kinds of industrial processes to specific, required values and to maintain said values as accurately as possible. Only then can reliable and economical operation of the respective technical system be ensured.
Alongside cited maintaining of the process metrics at required values, the dynamics of the control mechanisms employed often also play a major role: On the one hand the controllers must intervene promptly; on the other hand the actuating signals they produce must exhibit required temporal characteristics. It is in particular usually necessary to avoid too great an overshoot and too fast an actuation. The latter problem is addressed by, for example, applying the actuating signal to the control element in a ramped manner; the actuating signal is for this purpose routed to what is termed a ramp generator, which can additionally perform control-output limiting. The ramp generator generates a further actuating signal (which, where applicable, is restricted in its amplitude in accordance with the above-cited required control-output limiting) which rises linearly up to the end value of the applied actuating signal and which is routed to the control element.
For example, in the case of a steam power plant the flow of steam mass to the steam turbine must be controlled according to specified, required characteristics in keeping with the specifications of the manufacturer of the steam turbine. It is usually necessary here to maintain an actuating speed at which the steam valve must be opened in response to a control output or, proceeding therefrom, must be closed. This means, for instance, that the steam turbine must not be subjected to a sudden impact of a large quantity of steam.
The steam valve can be embodied in the form of, for instance, a valve which is actuated via a drive driven in turn via a servo valve. Control current is here used as the actuating signal for the servo valve. This current signal is generated in known control mechanisms by further processing the output signal (customarily having a determined fixed value for achieving a required flow of steam mass to the steam turbine) of the valve positioner in the steam turbine controller into the actuating signal for the steam valve by means of a previously cited ramp generator. The rate of rise of the actuating signal generated by the ramp generator of the steam turbine controller and said signal's possibly required control-output limiting have hitherto not be specified until the power-on phase of the steam turbine controller in the steam power plant. Alongside a servo drive for opening and closing, the steam valve has a position sensor which reports the present position in terms of its degree of opening to the steam turbine controller in the form of a current signal.
For setting a required opening or, as the case may be, closing speed of the steam valve, said valve is usually opened and closed several times for test proposes when the steam turbine controller is being powered on in order to assign the current signals then generated by the position sensor of the steam valve to the respectively current degree of opening of the steam valve. For example, the steam valve is taken from the fully closed to the fully opened position within a period of 1-5 sec. and the current signals generated during this process by its position sensor are registered. As an example, for the fully closed position a current of 4 mA could be produced which thereafter rises for example essentially, or otherwise according to other temporal characteristics, linearly up to an end value of, say, 18 mA corresponding to the fully opened position.
The current characteristic obtained during power-on is then implemented in the steam turbine controller's ramp generator so that, in the event of operation, the steam turbine valve can be opened and/or closed within a required period of time, meaning, therefore, that a valve-actuating speed required by the steam turbine manufacturer is maintained.
A disadvantage of this known method is that the steam turbine controller takes a long time to power on as the actuating signal for the steam valve first has to be matched locally in the power plant to the required temporal characteristics, for example to a defined valve-actuating speed, and a test phase must, in particular, be provided for this.